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Chain Barrel in Loam.                            6 5

In the propeller which we moulded in our last chapter
(Fig. 35) a screw template was placed outside the mould. There
are?cases, however, when a screw is to be moulded in loam, but






where the  course mentioned cannot,  for   certain  reasons,  be
followed.    Such is shewn at Fig. 73^.

A Chain Barrel for a crane is formed with a helical groove
to receive the chain. A is the casting required; B shews the
striking out of the loam, and c the finished mould. The only
portion requiring explanation is the screw </, made of hardwood.
It is fixed to the bottom plate, and has the same pitch as the
chain groove, though, of course, a more abrupt rise (for this
reasc>n made as large as convenient). The striking board runs on
the screw, being supported by a roller, and balanced by weight, as
in the case of the propeller.